1. Technical Field
The present invention relates to a physical quantity sensing unit equipped with a capacitive sensor and a physical quantity sensing method carried out in the sensing unit, which are directed to sensing physical quantity such as acceleration, and in particular, the physical quantity sensing method and unit that are able to perform self-diagnosis against a sticking phenomenon caused between electrodes incorporated in the capacitive sensor.
2. Related Art
In recent years, people's consciousness for safety of automobiles has been increased noticeably, so that an airbag system has been mounted on vehicles as standard equipment. Under such a situation, a great deal of demand for compact acceleration sensors of higher accuracy and higher reliability has arisen as a sensor for sensing vehicle's collisions.
The acceleration sensor includes various types of sensors depending on how to detect acceleration, such as piezoelectric type of sensor and capacitive type of sensor. The mainstream is the capacitive type of acceleration sensor, because the sensor can be manufactured with some advantages, such as higher detection sensitivity and less temperature drift.
A capacitive type of semiconductor-made acceleration sensor is provided as one type of such sensors. This acceleration sensor is equipped with a pair of fixed electrodes and a movable electrode disposed to face each fixed electrode between the fixed electrodes. The movable electrode serves as a movable member which can be displaced depending on amplitudes of acceleration to be applied to the sensor. A difference capacitor is formed between the two variable capacitors each of which composed of the movable electrode and either fixed electrode. Changes in the capacitance of the difference capacitor are detected as signals indicative of the acceleration to be applied.
By the way, in cases where such a capacitive type of sensor receives acceleration of an excessive amplitude or is in the process of manufacturing, there may occur a phenomenon called sticking phenomenon that makes it impossible or difficult to unstick the movable electrode from a fixed electrode after the movable electrode has stuck to the fixed electrode. In a stuck state of the movable electrode to a fixed electrode, detecting the acceleration with precision is difficult. Hence it is required to perform a self-diagnosis for possible sticking phenomena in recognizing whether or not the normal detection can be carried out.
Some types of sensors to cope with the above difficulty have already been proposed. One type of sensor, which is proposed by Japanese Patent Laid-open No. 8-110355, is a capacitive type of sensor that has a function of determining that an acceleration signal outputted from the sensor shows a true value thereof, have experienced a sensor's erroneous detection, or results from an erroneous operation of an electrical apparatus in charge of evaluating the signal.
Another type of sensor, which has already been proposed by the present applicant in Japanese Patent Laid-open No. 2002-040047; has already been realized as an angular velocity sensor. This sensor has a self-diagnosis function to diagnose whether or not the sensor outputs an accurate signal, without requiring pads dedicated to the self-diagnosis or a further electrode dedicated to generating an electrostatic attraction.
However, the foregoing conventional sensors still have a drawback that the self-diagnosis owned by the sensors is permitted to be performed only when the movable member is displaced to the extent that an output signal from the sensor is limited within a certain level. In other words, it is an unexpected circumstance for the conventional sensors to have reception of an excessive acceleration, so that the conventional sensors cannot be tested with regard to whether or not the movable member is able to surely return to its original position when the movable electrode is largely displaced to touch with a fixed electrode.
In addition, observing the sensor states before and after the application of an excessive acceleration may be performed under a hammering test, which actually gives an excessive acceleration to a sensor. This test faces, however, some difficulties because as an apparatus for the test becomes larger in its size, the test takes a negligible period of time because it is mechanically carried out. Hence, in practice, it is almost impossible to test all the sensors produced in a large scale.